FIELD OF THE INVENTION
The present invention relates to a method for making metal oxide aerogels and porous glasses. In particular, the invention is a two-step process in which a condensed metal oxide intermediate is formed, and from which aerogels are made having any selected density, with high clarity, thermal insulative capacity, moisture stability, and strength.
Metal oxide aerogels are ultra-light, highly porous forms of metal oxide glass, with densities only slightly greater than air. The first aerogels, made in the 1930's, were scientific curiosities whose unique and strange properties were not immediately exploited for practical purposes. Decades later, aerogels found a practical application in a highly specialized area--the detection of charged particles in high-energy physics experiments. Today, the potential applications for aerogels are numerous and diverse. Their unique properties make them especially useful for a variety of applications that require transparency, low thermal conductivity, and strength with very low weight.
Aerogels are generally transparent and have a thermal conductivity about 100 times less than conventional non-porous glass. Because of their transparency and excellent insulating properties, aerogels could be used as superinsulating materials in walls, windows, refrigerators, boilers, boiler houses and steam pipes, or passive solar collectors. Aerogels also have mechanical strength and are good sound and shock absorbers. Sound transmission through aerogels is slower than through air, and the acoustic impedance of aerogels falls between that of most sound transducers and air. Aerogels could be used to improve the efficiency of transducers used in micro-speakers and distance ranging. Aerogels have low dielectric losses and would make excellent substrates and supports for electronic circuits, especially microwave.
The extremely low densities of aerogels, coupled with their mechanical strength, suggest a host of applications as materials for engine and body parts of automobiles, aircraft, and spacecraft. Aerogels would be useful as packaging materials and have the added advantage of being environmentally friendly and non-toxic, unlike typical plastics and Styrofoam, which release CFCs. Aerogels also have a smoky, ghostly appearance and can be colored with dopants, lending an aesthetic quality to the materials that is desirable for more artistic uses, such as novelty or craft items, and toys. These dispersed dopants could have useful applications in which the aerogel acts as a host matrix. Examples include dye doped aerogels, which could be used as laser rods, and metal doped aerogels, which could catalyze specific chemical reactions.
Two general reactions are used to make metal (M) oxide aerogels: ##STR1##
A metal alkoxide is hydrolyzed by reacting with water and an alcohol in the presence of a catalyst. The hydrolyzed metal undergoes a condensation reaction, forming a metal oxide gel, from which solvents are extracted to form an aerogel.
The first aerogels were translucent pieces of porous silica glass made by S. S. Kistler (Nature 127:741 (1931); U.S. Pat. No. 2,249,767). The aerogels were prepared by forming silica `hydrogels`, which were exchanged with alcohol and dried with little shrinkage. When the alcohol was supercritically extracted from the wet gel at high temperatures and pressures, the resulting aerogel had a density of about 0.05 g/cm.sup.3, or 98% porosity. Kistler's process was time consuming and laborious, and subsequent advances in the art have reduced the processing time and increased the quality of aerogels.
An improvement over Kistler's method was described by Teichner et al. (U.S. Pat. No. 3,672,833). The single step sol-gel process uses a silicon alkoxide such as tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS). The silicon alkoxide is hydrolyzed by one to ten times the stoichiometric quantity of water with an alcohol solvent in an acid, neutral, or basic medium. The hydrolysis reaction is followed by condensation, in which the hydrolysis products polymerize to form a gel.
In Teichner's method, the wet gel contains reaction-generated alcohol, and therefore the slow process of exchanging solvents before drying, as in Kistler's method, is unnecessary. The alcohol is removed directly from the wet gel by exposure to temperatures and pressures above the alcohol's supercritical point. These conditions re-esterify the aerogel surfaces, making the material hydrophobic and stable when exposed to atmospheric moisture (U.S. Pat. No. 2,680,696 by E. C. Broge).
In Teichner's method, the wet gel contains reaction-generated alcohol, and therefore the slow process of exchanging solvents before drying, as in Kistler's method, is unnecessary. The alcohol is removed directly from the wet gel by exposure to temperatures and pressures above the alcohol's supercritical point. These conditions re-esterify the aerogel surfaces, making the material hydrophobic and stable when exposed to atmospheric moisture (U.S. Pat. No. 2,680,696 by E. C. Broge).
The silica aerogels made by the Teichner process have improved properties of transparency and strength over those produced from Kistler's method. However, the aerogels produced by the method of the present invention have an even greater range of densities and substantially improved physical properties of clarity, strength, moisture stability, and insulative capacity.
S. Henning and L. Svensson further improved aerogel synthesis methods by developing a commercial process (Phys. Scripta 23:698 (1982); U.S. Pat. No. 4,402,927 by von Dardel et al). A silicon alkoxide, tetramethoxysilane (TMOS), is reacted with water in the presence of a basic catalyst (NH.sub.4 OH) in a single mixing step: ##STR2##
The condensation reaction immediately follows hydrolysis in the same reaction vessel.
Conventional silica aerogel glasses have distinguishable microstructures that are characteristic of the particular reaction process used for their formation. The microstructure of the aerogel made by the Henning process is composed of spherical primary particles linked to form a `colloidal` network resembling strands of pearls randomly crosslinked together and surrounded by the reaction-generated alcohol. The reaction rates of the hydrolysis and condensation steps ultimately determine the microstructure of the gel (R. K. Iler, The Chemistry of Silica, Wiley Interscience, New York, 1979, and D. W. Schaefer, Science 243:1023 (1989)). The reaction rates, in turn, strongly depend on the pH, which is adjusted by the addition of base catalyst.
Aerogels made by conventional techniques have a maximum bulk density limit. Conventional silica aerogels made by the single-step hydrolysis-condensation reactions given in Equations [3] and [4] have densities in the range of 0.05-0.27 g/cm.sup.3. The density range of conventional aerogels described in other references is about 0.04-0.3 g/cm.sup.3 (Teichner et al., U.S. Pat. No. 3,672,833; von Dardel et at., U.S. Pat. No. 4,402,927; and Zarzycki et al., U.S. Pat. No. 4,432,956). Stoichiometric and miscibility considerations limit the expected maximum bulk density of silica aerogels to about 0.3 g/cm.sup.3.
In conventional practice, aerogels of low densities are obtained by dilution of the initial condensed metal oxide solution with alcohol. However, the higher the dilution, the longer the gelation time. Dilution considerably slows the hydrolysis rate, which significantly increases the overall gelation time. At some maximum dilution level, the reverse equilibrium reactions will inhibit gelation, thereby setting the minimum density limit for low density aerogels. To form an aerogel with a density of 0.02 g/cm.sup.3 using such a dilution method, as many as 14 days is required for the solution to gel completely. The present invention improves on the conventional techniques by forming a partially hydrolyzed, partially condensed metal intermediate, which can produce aerogels with densities extending beyond the conventional limits, with significantly shorter gelation times.
Aerogels are generally transparent when prepared by the conventional single-step method. However, a loss in clarity occurs in aerogels having a density less than 0.04 g/cm.sup.3. This loss of transparency in low density aerogels is caused by light scattering from pores that have diameters greater than 100 nm (visible wavelengths).
Tewari et al. describe an improved process for making silica aerogels in a single-step hydrolysis-condensation reaction of silicon alkoxide to form an "alcogel" (U.S. Pat. No. 4,610,863, "Process for Forming Transparent Aerogel Insulating Arrays"). The alcohol generated in the reaction is removed by substitution with liquid CO.sub.2, which is then removed by supercritical drying of the alcogel. Tewari et al. suggest that substitution of CO.sub.2 for the alcohol solvent will allow removal of solvent at less severe conditions of temperature and pressure.
The single-step Tewari process produces aerogels containing 5% silica, which corresponds to a density of 0.11 g/cm.sup.3. The chemistry of the Tewari method limits the attainable aerogel density range from about 0.02 g/cm.sup.3 to 0.3 g/cm.sup.3. Also, supercritical extraction of CO.sub.2 solvent following exchange with alcohol produces an aerogel with hydrophilic surfaces. Hygroscopic attraction of moisture to the surfaces of the aerogel leads to instability and eventual collapse of the aerogel structure if exposed to atmospheric moisture.
The microstructure and chemical properties of the dried aerogel are determined by its precursor chemistry. The precursor chemistry is controlled by several variables: the use of catalysts to adjust the pH of the reacting solutions, the amount of water used in the reactions, the use of additives to control polymerization rates, and the reaction sequence. For example, the microstructure of a single-step, base catalyzed hydrolysis-condensation of silicon alkoxide is a colloidal gel, whereas the acid catalyzed reaction leads to a polymeric gel.
The importance of the reaction sequence was demonstrated by Brinker et al., using a two-step process for making silica gels from which high density "xerogels" resulted after evaporative drying (J. Non-Cryst. Sol. 48:47 (1982)). The first step of the Brinker process involves the acid catalyzed hydrolysis of silicon alkoxide using a substoichiometric amount of water required to fully hydrolyze the silicon alkoxide. This first step produces a partially hydrolyzed, partially condensed silica in alcohol solution, in which the presence of alcohol limits continued condensation by affecting the reverse equilibrium reactions. The sol from this step could be characterized as consisting of clusters of polymeric silica chains.
The second step involves the base catalyzed completion of the hydrolysis-condensation reaction, where condensation continues until gelation occurs. The microstructure of the final gel made from this two-step process is more highly cross-linked and generally stronger than a single-step process gel.
Schaefer et al. describe a modified two-step process in which some of the reaction-generated alcohol of the first step is removed from the reaction by distillation, leaving a partially condensed silica intermediate (Physics and Chemistry of Porous Media II, J. R. Banavar, J. Koplik and K. W. Winkler, Eds. AIP New York (1987), pp. 63-80). The intermediate is dissolved with another alcohol before completing the hydrolysis-condensation with a base catalyst. The alcohol in the product gel is supercritically extracted, producing an aerogel. The microstructure of the resulting aerogel is polymeric, as in the acid catalyzed, single-step hydrolysis-condensation reactions.
Neither Brinker nor Schaefer recognize the adverse effects of the alcohol solvent on the hydrolysis-condensation reactions, which are due to re-esterification of the hydrolyzed species in the presence of excess alcohol. The presence of alcohol in the reactions affects the reaction rates and the degree of polymerization of the condensing gel, which also affects gel morphology. The presence of alcohol limits the gelation process to preclude the formation of aerogels of very low densities, less than 0.02 g/cm.sup.3. The present invention provides a method of producing aerogels with densities less than 0.02 g/cm.sup.3 by removing reaction-generated alcohol and introducing a nonalcoholic solvent to prevent the reverse reactions.